The Jackson Laboratory provides a unique resource for discovering new mouse mutations that have potential importance in the biomedical research. Genetic mapping of these mutations is a necessary first step in their molecular definition. We propose to improve the speed and efficiency of our genetic mapping methods. The first major goal of this application is to identify and map a complementary set of loci that can be detected by using only a few select multilocus DNA probes robes that detect multiple loci (gene-pseudogene or retrovirus families) on many chromosomes. Once characterized and mapped, these loci will provide many reference markers distributed throughout the mouse genome. The second major is to use these multilocus probes to rapidly map each new mouse mutation with a single cross and a minimum number of DNA blot hybridizations. Chromosomal localizations of many spontaneous mutations will inevitably identify several candidate genes whose dysfunction could result in the observed mutant phenotypes. The third goal of this project is to investigate the molecular @basis of those spontaneous mutations that map near candidate gene loci. Molecular definition of a phenotypic mutation will greatly increase its usefulness for biomedical research.